1. Filed of the Invention
The present invention relates to a receiving optical sub-assembly, in particular, the receiving optical sub-assembly having the so-called co-axial package.
2. Related Prior Arts
An optical sub-assembly installs a semiconductor optical device such as photodiode and laser diode therein and provides an optical aligning mechanism including a condenser lens, a sleeve, and so on to couple these devices to an optical fiber. The optical assembly whose transmission speed is below 2.5 GHz typically provides the co-axial package, except for special purpose assembly that builds in a temperature control function. Even the optical assembly for 10 Gbps transmission speed, which recently increases its reality, the package thereof usually applies the so-called butterfly package with an enough inner space for mounting the semiconductor devices and some electronic devices. However, to provide one solution for continuous request to make the assembly compact and low-cost, it has been investigated to apply the co-axial package for the optical sub-assembly used in the 10 Gbps transmission.
In the co-axial package, for instance, a transmitting optical sub-assembly (TOSA) mounts a laser diode (LD) on a metal stem via an insulating sub-mount, and electrically connects an electrode of the LD to a lead pin passing through the stem with a bonding-wire. To drive the LD, a modulation signal with a swing voltage about 1 V is necessary to drive the LD. While, the receiving optical sub-assembly (ROSA) directly outputs a photocurrent corresponding to an optical signal and generated by the PD, or outputs an electrical signal converted from the photocurrent and amplified by a pre-amplifier installed within the ROSA.
The Japanese Patent application published as JP-H07-312430A has disclosed one type of the ROSA which configures with the co-axial package and builds the PD and the pre-amplifier therein. On a center of the stem is mounted with a die capacitor and the PD is mounted on the upper electrode of this die capacitor. A signal from the PD is conducted to the pre-amplifier by the bonding wire directly connecting the PD with an input electrode of the pre-amplifier. The pre-amplifier is configured to convert the current signal into an electrical signal, to amplify thus converted electrical signal and to output the amplified signal from an output electrode thereof to the lead pin with a bonding wire. The output signal from this ROSA, whichever the current form and the voltage form, the magnitude thereof is far smaller than that of the TOSA, in the case of the voltage form, the magnitude of the signal output from the ROSA is generally smaller than 0.5 V.
It is quite hard to carry a signal, which is small enough in the magnitude thereof and as fast as 10 Gbps speed, on the load pin passing. Accordingly, the pre-amplifier is built within the sub-assembly to amplify the signal from the PD to output thus amplified signal, which is generally adopted in the optical assembly as shown in the Japanese Patent published as JP-H07-312430A. However, even the sub-assembly builds the pre-amplifier, a subject to satisfy the impedance matching condition for the lead pin is still left unsolved. Rather, because the sub-assembly builds the pre-amplifier in a limited space within the package, interconnections of the ground line and the power supply line brings difficulties, which causes peaks and dips in the frequency response, and occasionally the self oscillation of the sub-assembly.
A conventional sub-assembly with the co-axial package mounts the PD on the stem via the sub-mount, and installs the pre-amplifier just beside the PD. The ROSA is necessary at leas four lead pins, namely, the power supply (VDD) for the pre-amplifier, the bias voltage for the (VPD), the signal (Out), and the ground (GND). When the sub-assembly is requested to be operable for a high speed signal over 10 Gbps, another signal (/Out) complementary to the signal (Out) may be generally provided. In the conventional sub-assembly, four lead pins except for the ground are arranged on a circle and on the center of this circle, namely, the center of the stem, is arranged with the PD. The pre-amplifier is placed between two lead pins and just beside the PD. The ground pin (GND) is in directly contact to the stem in the back surface thereof.
FIG. 9 shows an arrangement of devices mounted on the conventional stem. This stem, typically made of iron or Kovar, has a disk shape with a diameter from about 3 mm to 6 mm and a thickness of about 1.0 mm. On a center thereof is mounted with the PD with a thickness thereof about 0.2 mm via the die capacitor. A light incident surface of the PD directs the normal of the stem. That is, although not shown in FIG. 9, the sub-assembly provides a lens above the stem, namely, a direction extending the normal of the stem. Optically alignment members are arranged so as to position the optical fiber in an opposite side of the stem with respect to this lens. Thus, the light emitted from the end of the optical fiber enters the incident surface of the PD through the lens.
The pre-amplifier is arranged in just beside the PD, in FIG. 9, between two lead pins beside the PD. Electrodes of the pre-amplifier are arranged in a line along both sides opposite to each other, one of which within one line is the electrodes for the signal (Out), and another electrode within the other line corresponds to the complementary signal (/Out). The power supply VCC for the pre-amplifier is supplied to the electrode on the pre-amplifier via another die-capacitor difference from that mounting the PD. The ground of the pre-amplifier is secured by wire-bonding the electrodes arranged along respective sides directly to the surface of the stem.
In the arrangement shown in FIG. 9, although the ground of the pre-amplifier may be secured by bonding the ground electrodes thereof directorly to the surface of the stem, a length of the bonding-wire is unavoidable to be lengthend by a length corresponding to a thickness of the pre-amplfier wire-bonded wherever the surface of the stem. When the assembly operates in 10 GHz range, the characteristic length sometimes shorter than 1 mm. While, the thickness of the pre-amplifier is generally about 0.1 mm to 0.5 mm, accordingly, the length corresponding to the thickness of the pre-amplifier reflects in the high frequency performance of the pre-amplifier in the 10 Gbps range.